This invention relates to a system and method of determining and controlling weld nugget penetration during resistance spot welding using an acoustic technique. Faulty welds due to lack of penetration can be detected and controlled.
Resistance spot welding setup procedures are often based either on using handbook values or trial and error techniques. Once production parameters have been determined, the spot welding machine current, heat and load are set on the machine controls. Test coupons are made periodically during production and destructively inspected to determine weld nugget size, penetration and strength. This procedure is costly and has lead to faulty welds ending up in production, since test coupons are not always representative of production welding conditions.
There are essentially four types of resistance spot welder process controls on the market and these are categorized according to the variable measured in the welding process. They are: (1) thermal expansion; (2) nugget electrical resistance; (3) electric power; and (4) other methods consisting of ultrasonic, acoustic and infrared type sensing devices. Among the latter is a method based on the stress wave energy emission signature which is compared to an acceptable signal to control the resistance welding process. An acoustic control system has been briefly described that stops weld current at the onset to expulsion. Another author describes a technique using ultrasonic signals which are transmitted through the workpiece during the welding process. The method is based on the effect that temperature and molten metal have on wave velocity and wave distortion. Signature analysis and comparison to a reference are then used to track nugget formation.
An object of this invention is to provide an improved adaptive control system and method which will assure high quality resistance spot welds based on sensing an interior variable such as liquid/solid interface formation. Most other techniques currently available try to obtain weld nugget information by measuring external or response variables such as thermal expansion, temperature, etc., or signatures of internal behavior.